This invention is directed to semiconductor memory devices and methods of making such devices, and more particularly to an improved MOS memory cell of the random access type.
Semiconductor memory devices are widely used in the manufacture of digital equipment particularly minicomputers and microprocessor systems. MOS RAMs have been selected for use in an increasing portion of this field. The number of bits in a chip has increased dramatically, going from the industry standard of 1024 in 1972 to 4096 today, and 16384 bit device will soon be in volume production. A "4K" or 4096 bit MOS RAM is described in U.S. Pat. No. 3,940,747, issued Feb. 24, 1976 to Kuo and Kitagawa, assigned to Texas Instruments. 16K RAMs are described in Electronics, Feb. 19, 1976, pp. 116-121 and May 13, 1976, pp. 81-86. These high density devices use one-transistor dynamic memory cells which have the advantage of very small size, but require sense amplifiers capable of distinguishing a few hundred millivolts, and dissipate considerable power in large arrays, and of course suffer from the disadvantage of requiring external refresh systems. The arrays must be addressed periodically to restore the data, since the stored voltages will leak off the capacitors in the memory cells. This imposes both time and hardware burdens on the system. Static memory cells of the type set forth in copending application Ser. No. 691,252, filed May 28, 1976, by G. R. Mohan Rao, assigned to Texas Instruments, avoid the need for refresh, but at the expense of larger cell size and increased power dissipation. Various types of "self-refreshing" cells have been proposed, one example of which is shown in Digest of Technical Papers, 1976 IEEE Solid State Circuits Conference, p. 132, on Automatic Refresh Dynamic Memory. Another is shown in U.S. Pat. No. 3,955,181, issued May 4, 1976 to Joseph H. Raymond, Jr., for Self-Refreshing Random Access Memory Cell, assigned to Texas Instruments. These cells provide apparently static operation in that refresh is accomplished without addressing the cells. Prior self-refresh cells, however, have exhibited either large cell size or excessive power dissipation, neither of which is compatable with high density memories of the 4K or 16K variety.
It is a principal object of this invention to provide improved memory cells in semiconductor integrated circuits. Another object is to provide an improved RAM cell for MOS memory devices. An additional object is to provide small area, self-refreshing memory elements in semiconductor integrated circuits, particularly with lower power dissipation.